When the ACLU first filed a lawsuit in the wake of the Proposition 8 passage I felt their legal arguments were weak. As it turns out I was just ignorant, they have smart lawyers over there.
The main question in the ACLU lawsuits, and similar suits filed is: whether proposition 8 is an amendment or revision to the California Constitution?
If the proposition is a revision to the constitution, then it's illegal. Constitutional revisions can only be placed on the ballot only by a two-thirds vote of the Legislature.
A constitutional revision is any change which touches upon the core elements of the document, as opposed to an amendment which is only tangentially related. In this case, there is a strong argument for revision because Proposition 8 would change the fundamental principle of equal protection.
Equal protection is the fundamental right to have our laws apply equally to all.
Since the Court already held that denying same sex couple the right to "marry" is a violation of equal protection even when they have civil-unions available, the current ACLU lawsuit is very strong.
They are so going to win this. Unless the CA Supreme Court does something stupid.
It recently occurred to me that arguing that gays don't need marriage since they have strong civil-union rights sounded a lot like another argument I heard before... separate but equal. That didn't turn out so well.